The disclosure of Japanese Patent Application No. 2001-313840 filed on Oct. 11, 2001, including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to a catalyst deterioration detection apparatus of an internal combustion engine, and a detection method performed by the apparatus. More particularly, the invention relates to a catalyst deterioration detection apparatus for detecting deterioration of a catalyst for purifying emissions from an internal combustion engine, and a detection method performed by the detector apparatus.
2. Description of Related Art
A catalyst for purifying exhaust gas is disposed in an exhaust passage of a vehicle-installed internal combustion engine. This catalyst has a capability of storing an appropriate amount of oxygen. If exhaust gas contains unburned components, such as HC, CO, etc., the catalyst causes oxidation of the unburned components using stored oxygen. If exhaust gas contains oxides, such as NOx and the like, the catalyst causes reduction of the oxides, and absorbs and stores oxygen produced by the reduction reactions.
The catalyst disposed in an exhaust passage is thus intended to purify exhaust emissions. Thus, the emission control capability of the catalyst is greatly affected by the oxygen storage capability of the catalyst. Therefore, the emission control capability degradation state of the catalyst can be determined based on the maximum amount of oxygen that the catalyst is able to store, that is, the oxygen storage capacity.
FIG. 11 indicates a relationship between the catalyst temperature and the oxygen storage capacity (OSC). More specifically, a curve (1) in FIG. 11 indicates a temperature-OSC relationship exhibited by a normal catalyst with respect to fuel having a low sulfur concentration. A curve (2) indicates a temperature-OSC relationship exhibited by a normal catalyst with respect to fuel containing a large amount of sulfur. Furthermore, a curve (3) indicates a temperature-OSC relationship exhibited by a deteriorated catalyst. A curve (4) indicates a temperature-OSC relationship exhibited by a further deteriorated catalyst.
As indicated in FIG. 11, the oxygen storage capacity OSC of the catalyst is dependent on the catalyst temperature THC. This dependence changes in accordance with the deterioration of the catalyst. For example, if a low-sulfur concentration fuel is used, a normal-state catalyst exhibits relatively great oxygen storage capacity OSC even at a relatively low catalyst temperature range as indicated by the curve (1). In contrast, if the catalyst is lightly deteriorated, the oxygen storage capacity OSC of the catalyst becomes relatively high in a relatively high catalyst temperature range as indicated by the curve (3). As the deterioration of the catalyst further progresses, the oxygen storage capacity OSC of the catalyst remains low regardless of the catalyst temperature. Therefore, it is possible to determine whether the catalyst is normal by checking whether a sufficient oxygen storage capacity OSC is secured when the catalyst temperature is, for example, within a range indicated by reference numeral (5) in FIG. 11.
However, it is known that the oxygen storage capacity OSC of the catalyst varies in accordance with variation in the quality of fuel used, more specifically, variation in the concentration of a sulfur component contained in fuel. That is, even if the catalyst is normal, a high sulfur component concentration in fuel causes a smaller oxygen storage capacity OSC as indicated by the curve (2) than a low sulfur component concentration (curve (1)) in fuel. In this case, in order to determine whether the catalyst is normal based on the oxygen storage capacity OSC, it is necessary to distinguish whether the catalyst is exhibiting the OSC characteristic indicated by the curve (2) or the OSC characteristic indicated by the curve (3). Therefore, if variations in the quality of fuels distributed in the market are taken into consideration, the case where the deteriorated state of the catalyst can be determined based on the oxygen storage capacity OSC is limited only to a case where the catalyst temperature is within a narrow range indicated by reference numeral (6).
The catalyst temperature changes in accordance with the amount of intake air. That is, if the amount of intake air is large, the catalyst temperature becomes high due to a large amount of high-temperature exhaust gas supplied to the catalyst. Conversely, if the amount of intake air is small, the catalyst temperature becomes low due to a small amount of flow of exhaust gas. Therefore, at a transitional time when the amount of intake air sharply increases, such as a time of acceleration of the vehicle or the like, there occurs a temporary event where the amount of intake air (amount of exhaust gas) becomes large while the catalyst temperature remains low.
If the state where the amount of intake air has increased continues, the catalyst temperature eventually becomes high, and enters the range (6). Even in that case, however, until the catalyst temperature sufficiently rises, there occurs a state where the amount of intake air is excessively large relatively to the catalyst temperature so that sufficient control of exhaust emissions (oxidation and reduction thereof) cannot be achieved, that is, there occurs a through-flow of exhaust gas. That is, during a period from a sharp increase in the amount of intake air until a sufficient rise of the catalyst temperature, an event similar to the event that occurs in the case of deterioration of the catalyst occurs even though the catalyst is actually normal. Therefore, during a certain time period from a transitional change in the operation state of the internal combustion engine until the amount of intake air and the catalyst temperature become unbalanced, it is preferable that the determination regarding deterioration of the catalyst be avoided even under a condition in which the catalyst temperature converges into the range (6) (first requirement).
If high temperature of the catalyst is caused by continuation of a travel in an accelerating state or with large amounts of air and is immediately followed by a travel with a small amount of air, exhaust gas may possibly be purified to a sufficient extent even though deterioration of the catalyst has considerably progressed. Therefore, it is preferable that determination regarding deterioration of the catalyst be avoided also in the case where there is unbalance between the amount of intake air and the catalyst temperature. However, if in the aforementioned case, deterioration of the catalyst has sufficiently progressed (see the curve (4)), the abnormality of the catalyst can be precisely determined, so that the detection should not be simply suspended (second requirement).
Japanese Patent Application Laid-Open No. 11-36848 discloses a catalyst deterioration detection apparatus that meets the aforementioned first requirement. This apparatus has a function of monitoring the rate of change of the amount of intake air, and of determining whether the vehicle is accelerating or decelerating or is in a steady running state based on the rate of change. This apparatus stores in a memory a lower limit-side criterion value and an upper limit-side criterion value that are to be compared with the rate of change of the amount of intake air. The criterion values are set so that the absolute values thereof increase with increases in the elapsed time following the beginning of an acceleration or deceleration of the vehicle. The aforementioned conventional apparatus permits execution of determination regarding deterioration of the catalyst only if the rate of change of the amount of intake air is between the upper limit-side and lower limit-side criterion values.
According to this conventional apparatus, during a short time period after a shift to acceleration or deceleration of the vehicle, that is, after a shift to a transitional state of the vehicle, determination regarding deterioration of the catalyst can be permitted only if there is substantially no change in the amount of intake air. If there is a great change in the amount of intake air, determination regarding deterioration of the catalyst can be permitted only in the case where a sufficiently long time has elapsed following the shift to a transitional state of the vehicle. That is, the aforementioned conventional apparatus is able to permit execution of determination regarding deterioration of the catalyst only if it can be estimated that the catalyst temperature and the amount of intake air are balanced, regardless of the magnitude of change in the amount of intake air. Therefore, the aforementioned conventional apparatus is able to accurately determine a state of deterioration of the catalyst based on the oxygen storage capacity OSC.
However, the above-described conventional apparatus tends to unnecessarily prohibit execution of determination regarding deterioration of the catalyst if small amounts of acceleration and deceleration are repeated (microscopic view) during a larger accelerating or decelerating state (macroscopic view).
During operation of a vehicle, there occurs a situation where small amounts of acceleration and deceleration are repeated (microscopic view) as indicated in FIG. 12B during a larger accelerating state (macroscopic view) as indicated in FIG. 12A. In this situation, the above-described conventional apparatus resets the elapsed time following the shift to a transitional state every time the vehicle shifts from an acceleration to a deceleration or from a deceleration to an acceleration in a microscopic view. Therefore, the apparatus experiences an event where determination regarding deterioration of the catalyst is not executed even though the amount of intake air and the catalyst temperature are balanced.
As described above, the conventional catalyst deterioration detection apparatus adopts a construction in which determination regarding deterioration of the catalyst is executed only under a severely limited condition, in order to determine deterioration of the catalyst with good precision. Therefore, the conventional catalyst deterioration detection apparatus is able to determine deterioration of the catalyst with high precision if the determination can be executed, but is not always able to execute the determination with a desired frequency.
Accordingly, it is an object of the invention to provide a catalyst deterioration detection apparatus and method capable of executing high-precision determination regarding deterioration of the catalyst with high frequency, and a detection method thereof.
In order to achieve the aforementioned and/or other objects, one aspect of the invention provides a catalyst deterioration detection apparatus and method for detecting deterioration of a catalyst provided in an exhaust passage of an internal combustion engine. A deterioration characteristic value that indicates a state of deterioration of the catalyst is determined. A temperature of the catalyst occurring at a time of the determination of the deterioration characteristic value is determined as a detection-time catalyst temperature. An amount of intake air occurring at the time of the determination of the deterioration characteristic value is determined as a detection-time amount of air. A determination is made as to whether the detection-time catalyst temperature and the detection-time amount of air satisfy a predetermined relationship. A state of deterioration of the catalyst based on the deterioration characteristic value can be determined. However, the determination of the state of deterioration of the catalyst based on the deterioration characteristic value is prohibited if the detection-time catalyst temperature and the detection-time amount of air do not satisfy the predetermined relationship.
According to the catalyst deterioration detection apparatus and the detection method thereof, even if the catalyst temperature and the amount of intake air becomes unbalanced, false determination of the state of deterioration of the catalyst caused by the unbalance can be prevented. Therefore, in this aspect of the invention, there is no need to limit the situation where determination of the state of deterioration of the catalyst is allowed, to a situation where such unbalance will not occur. Therefore, according to this aspect of the invention, determination regarding deterioration of the catalyst can be performed with good-precision and high frequency.
In accordance with another aspect of the invention, a catalyst deterioration detection apparatus and method detects deterioration of a catalyst provided in an exhaust passage of an internal combustion engine. A deterioration characteristic value that indicates a state of deterioration of the catalyst is determined. A temperature of the catalyst occurring at a time of the determination of the deterioration characteristic value is determined as a detection-time catalyst temperature. An amount of intake air occurring at the time of the determination of the deterioration characteristic value is determined as a detection-time amount of air. A target catalyst temperature corresponding to the detection-time amount of air is calculated. An amount of separation between the detection-time catalyst temperature and the target catalyst temperature also is calculated. A state of deterioration of the catalyst based on the deterioration characteristic value can be determined. However, the determination of the state of deterioration of the catalyst based on the deterioration characteristic value is prohibited if the amount of separation is greater than a predetermined amount.
According to the above-described catalyst deterioration detection apparatus and method, the unbalance between the catalyst temperature and the amount of intake air can be determined based on the magnitude of amount of separation between the detection-time catalyst temperature and the target catalyst temperature. Then, using the result of determination, determination regarding deterioration of the catalyst can be performed with good precision and high frequency.